Amplifying coupling between a low resistance source and a high resistance circuit



Patented Apr. 6, 1948 All/IPIQIFYING COUPLING BETWEEN A LOW RESISTANCE SOURCE AND A HIGH RE- SISTAN CE CIRCUIT `William H. Bussey, Chicago, Ill., assignor, by mesne assignments, to Robert L. Kahn, trustee Application May 29, 1945, Serial No. 596,503

i f l This invention relates to `an electrical apparatus and more particularly to an amplifying coupling between a low resistance low voltage source of current and a highresistance relatively high voltage circuit.

There are many low resistance low voltage sources of power whose output it is desired to utilize for recording, indicating or control pur-` poses in general. Thus, thermal couples forming pyrometer systems, barrier type light cells, and even bridges and potentiometers'under some conditions may be classed as low resistance low voltage sources of current. ASuch source generally has a voltage output whichi-s measured in millivolts with currents measured vinl milliamperes. The resultant power output vis of Vthe microv.'att order.

The voltage output Aoi" low voltage low resistance power sources previously mentioned does not vary at a rapid `rate. Thus, a thermal couple whose voltage output is proportional to a temperature difference may vary itsV voltage output as the temperature varies. However, even` by such lowfrequencystandards as 60 cycles per second, the `variation of voltage of such a thermal couple is extremely small., -lt` is clear, therefore, that if the voltage output of 'such low resistance low voltage sources is to be amplified-direct current amplifyingl ucircuits must "be relied upon. t

Direct current ampliersfor `low voltage power sources suffer from serious drawbacks'.` Due to the necessity for continuous metallic circuits, excepting through the inter-electrode space in vacuum tubes, it is impossible 'to `provide any potential isolation for various-portions ofan amplifier. Contact potentials generated at'various junctions in an amplifying systemand usual tube disturbances result in drifts ofpotenti'als of the order ofthe potential output offthe low resistance sources. It is clear, therefore, that feeding a low voltage output from power sources previously described directly intocontrol elements of conventional direct current amplifiers is impractical.

By virtue of the inventionherein, it is an object to provide a simple and effective means for coupling a low'voltage low resistance power source to any conventional amplier. The invention is 7 Claims. (Cl. 179-471) cathode. A characteristic curve for a diode generally has a lower and upper bend. At the upper bend of the curve, saturation takes place so that, beyond a certain anode potential, space current remains either constant or increases relatively little, assuming no ionization of remanent gas particles. It is also possible to vary the space current by maintaining the anode potential at a saturating value and vary the temperature of the emitting cathode. The invention herein relies upon this latter property.

In accordance with the invention herein, there is provided a pair of diodes so connected that the voltage output from a low voltage low resistance power source unbalances a predetermined distribution of cathode heating current in said diodes. The unbalance results in one diode cathode having its heating current increased., while i the other cathode has its heating current decreased. By virtue of this, the electron emission is differentially affected, thus resulting in a diiferential variation of anode to cathode current in each diode. It is understood that the potential applied to both anodes is -suicient to maintain saturation in both diodes under all normal operating conditions but below that at which any arc or gas discharge may be initiated. l

By utilizing such a doublediode arrangement in a null control, the operating range of the diode, both as regard cathode temperature and anode bias, may be made quite small. By virtue of such an arrangement, the cathode emission and anode potential may be maintained `at; predetermined calibrated values. However, the system may be used without the null expedient.

The diodes are disposed in a conventional load circuit having a load resistor for each diode. The variation in space current through load resistors results in a variation of, potential drop across said resistors. `Such drops are large in comparison to the magnitude of the original low voltage power source. Usingnull operation, it is evident that the potential drops across the load resistors may be the basis` for this method of regulation. By causing such drops to retain their predetermined values, the system may be maintained in a balanced condition. On the other hand, if the system is permitted to respond to the original voltage from the low resistance power source without any attempt to balance the same out, the large potential drops across the load resistors may beV fed to conventional amplifiers. Such large drops may be as much as one thousand times as great as the original low potential, so that undesired disturbances in the amplifier will no longer be significant. By using` diodes that are as near alike as possible, disturbances within the diodes will generally tend to cancel out.

Under certain conditions, the diodes may be energized by alternating potentials having a peak lvalue suilici'ent for saturation under all normal operating conditions. The frequency ci this alternating potential should be substantially higher than the frequency of any normal potential variation present in the low resistance source. Thus, the low voltages may be used for modulation pur-v poses and the resulting output be fed through coupling condensers to conventional ampliiiers.

While the invention may be used with advantage particularly in connection with low voltage low resistance sources, it is by no means restricted to this. Thus, low voltages from high resistance or impedance sources may be utilized. En such a case, it is clear the null method for balancing out potentials would be necessary to eliminate a tendency for drawing any current from such high resista-nce sources. However, it is preferred to use the system in connection with low resistance sources so that currents having a value significant in comparison-to the value of the heating current in the cathodes may be drawn even ii only for a short time. Y

The two diodes are preferably within one envelope and are vas alike as possible, both in their construction and operating conditions. The cathodes themselves are preferably of the filamentary type since manufacturing diiculties would make an indirectly heated cathode expensive and of doubtful value.y In general, a variation of heating voltage of about one per cent can cause a significant variation in electron emission of a cathode.y Under certain conditions, the percentage variation of voltage may be even smaller. It follows, therefore, that lamentary cathodes operated at low voltages with minute heating currents are advantageous. Such a cathode has the advantage that it may be energized by one or more dry cellswhose 'output voltage may be maintained at aconstant value. It is, of course, possible to-energize the cathodes with alternating current or from any other power source providingv that the average value is maintained constant; In general, however, an alternating current heating source Will result in4 temperature alternations which may beobjectionable.

Filamentary cathodes may be made oi wire or any suitable. material and treated with suitable compounds for electron emission. Thus, it is possible to provide platinum wire having a neness of the order of one-tenY thousandth of an inch. Such a platinum wire may be coated with suitable ccmpoundslior promoting electron emission at high temperatures. There are also on the marketv small vacuum tubes particularly adapted for hearing aids having lamentary cathodes drawing twenty milli-amperes at about iive-eighths of. a volt. Such tubes may be operated as diodes and used satisfactorily. For a further description of the invention, reference will Vnow be made to the drawing wherein the single gure shows a diagrammatic representation of a system embodying the invention. i

Vacuum tube I has cathodes I2 and I3 preferably formed as one continuous length of wire. Disposed around each ofthe cathodes are anodes 'I4 and I5 respectively. Inasmuch as the construction of a double diode iswell known in the art, no detailed structural description will be given'. If desired, the two diodes may be sepa rated into two` separate structures,

, Betweencathodesl I2 and; I3; junction II may 4 be provided. This point is preferably located at the mid-point of the two cathodes so that the resistances of cathodes l2 and I3 are equal.

Cathodes I2 and I3 have leads I8 and I9 going toa steady source of heating; current 2-0. This source of heating, current `slzouldJh-ave aconstant power output and, in most instances, may consist of a battery. Bridged across leads I8 and IS are resistances- 2.| and 22 with junction 23 at the midpoint; Besistances 2| and 22 are preferablyL equalin value to the resistances of filamentaryfcathodes I2 and I3 under normal operatingconditions. Y

From junction: 23; a lead goes to terminal 24 While, from junction I'I, a lead goes to terminal 25, these terminals constituting input terminals to the system. Terminal25 is connected by lead 26 to any low voltage source of power 21 as previously described. tiource` 2lx has lead-'28 connecting it to.- neutralizing low voltage source 29 and then the circuitcontinues along-lead 32'back to terminal 24. Neutralizing` low voltage source 29 is provided for balancing out any low voltage from source 2'l so:` that the netV potential difference between input. temiinals. 2-4: and 25 may be reduced to zero. The; neutralizing; low* voltage source may consistrv ot a batt-ery andv potentiom` eter. The means for varying the. value Vof the neutralizing low voltage may be: either manual or automatic and,v if the: latten vmay. 'be operated from the output oi the; entire system.r If desired, the neutralizing lowvoltage source may be omitted and low voltage, source: 21 connected directly across terminals 24 and 2:5;

Anodes I4 and I5 are connected by leads 3 5 and Soto load resistors 31r and 38frespectively- Load resistors 31 and 38` have a value substantially equal tothe static-resistance'ofY eacha diode under normal operating conditions.` Load resistances Blandv 38 are connected' to junction point 39. From junctiony 39 a connectionV goes to high potential sourceI 40. This may consist of a battery orrectiied power'supply with the positive terminal connected: to, junction 39.y The negative terminaloi thehiglr volta-ge supply is connected by lea-dlt to junction point; Hr., The voltage of potential sotuceidllr is suiliciently high to maintain.- saturation init-he diodes under normal operating conditions=..- Under cer-tain conditions, voltage'source 40.mays be an alternating potential of a suitable-.high-valueand preferably havingauniformpeak voltage; f

Bridged acrossI leigh,.voltageb source Ml maybe resistance 42. with: which wiper 43: cooperates to provide a` potentiometer. Wiper 43 is groundedso that any desired potential? `level in` the-high voltage supply may be establishedasf ground. ThisA ground connection isv provided for'thecon,- venience. of the amplifier into: which the system may feed. The. ground-may., of: course, bef omitted andY other portions; of the systemj may be established-as groundfiidesired.A Y

Leads 35 and 3.6 pass` on to controlgrids 45 and 46- of vacuumv tube` ampliiers` 4'I.-V and 4%. These v amplifiers are merely Vrepresentative and may be; varied: to-sutcondi-.tionsa Thusas shown, vacuum tubes; 412 and; 4&- haaien cathodes,v 119i` and 5.0.- connected together tlnfougfh-v suitablef bias.-resist ance 5I,tc ground, By virtue of the establish:- ment of theground connectionV at,V a potential equal-.to or near the'positive;potentialoi source 40, the potentials of gridsf 52 and 415- may be readily established at. a; suitablef'value ,with ree spect to cathodes" 49.11 audio; without; elabora-teV means. y Y

Tubes lil and 48 have anodes 53 and 54 connected through load resistors 55 and 56 to a suitable source oi B plus potential. This potential may either be direct or alternating as desired. Output leads 5l and 58 taken at anodes 53 and 54 may feed succeeding amplifiers or suitable indicating or operating means. Thus, output leads 5l and 58 may control servo-means for varying neutralizing low voltage source 29 to maintain the system in balance.

In the event that the anode to cathode potential on the diodes is alternating, coupling condensers between control grids 45 and i6 and the diode anodes may be inserted. In such a case, it is clear that conventional amplier stages be used instead of direct current ampliiiers.

It is possible to couple an indicator to the con trol of neutralizing low voltage source 2d with the indicator calibrated to show the voltage necessary to balance the output from low voltage 2l. The indications may also be in terms of temperature or any desired characteristic. It is clear that the variation of neutralizing low voltage to maintain cathodes l2 and I3 balanced involves a null method. Such a method is highly advantageous since departure from carefully calibrated conditions with regard to cathode temperature and anode potential is not necessary. However, .in the event that the null balancing is not desired and voltage source 21 feeds directly into terminals 2Q and 25. then other indicating means will be necessary. Such indicating means may be acw tuated by the outputs from leads 5l and 58.

What is claimed is:

1. In a system of the character described, vacuum tube diode system having two cathodes and two anodes, a heater circuit for energizing each cathode to electron emission, electric connections disposing said heater circuits in series to provide a common heater terminal and two end heater terminals, a steady source of potential connected to said end terminals, said steady potential source having a neutral point'l normally at the same potential as the common terminal of said heater circuits, said system being adapted to have a low resistance low voltage source of power connected between said neutral point and common terminal, a cathode anode circuit for each diode including a resistive load and source of steady potential, said two cathode-anode circuits being similar, said system having an output terminal at the end of each resistive load adjacent a diode electrode, said last-named steady potential being high enough to maintain saturation conditions.

2. In a system of the character described, a pair of diodes each having a lamentary cathode and anode, a common connection between said filaments so that said filaments are in series, a direct current source connected across said two series filaments for supplying heating current, said direct current source having a neutral point whose potential is normally substantially equal to the potential of the common filament connection, said neutral point and common connection being input terminals for a low resistance low voltage source of power whose output po tential is measured in milli-volts and whoseout put current is normally measured in milli-am-1 peres, said laments being so designed that they are energized by a heating current normally meas ured in milli-amperes at a sufciently low volt-- age so that upon insertion of said source of power said heating currents are sensibly alected and result in a significant change in electron emission, two resistive loads in series connected across said anodea a source of steady direct potential having its `negative terminal connected to said common filament connection and its positive terminal connected to the common connection bee tween said resistive loads and an output circuit across said anodes, said steady potential being high enough 'to maintain saturation conditions.

3. The system of claim 2 wherein a ground is taken at a point in said source of saturating potential having a potential near the positive terminal whereby output terminals may be at potential sufficiently close to ground so that connection to conventional vacuum tube ampliners is possible.

el. The system of claim 2 wherein a point on said source oi saturating potential is grounded so that the positive terminal is at or near ground, a pair of opposed vacuum tube ampliers and direct connections from said output terminals to the control electrodes of said vacuum tube arnplifiers.

5. In a system of the character described, a vacuum tube diode system having two filamentary cathodes and two anodes, a series connection between said laments to provide a common iilament terminal and two end filament terminals.

a steady source of direct potential connected to said end filament terminals for heating said filaments in series, said source having a neutral point whose potential is intermediate the potentials of the end filament terminals, said neutral point and common filament terminal constituting the input terminals for the system and being adapted to have a low resistance, a low voltage source of power connected thereto, two resistive loads connected in series between the diode anodes, said loads having a common terminal, a second source of potential connected between said load common terminal and common filament terminal and being high enough to maintain diode saturation conditions, said system having output terminals at the diode anodes whereby a potential at the input of the system differentially changes the distribution of heating current in said two filaments and thereby differentially changes the space currents through said diodes.

6. The system of claim 5 wherein said steady source of direct potential for heating said laments includes a resistor connected across said end lament terminals and wherein said neutral point is an intermediate point on said resistor.

7. The system of claim 5 wherein said steady source of direct potential for heating said 1a REFERENCES CITED The following references are of record in the file oi this patent:

UNITED STATES PATENTS 

